Carbohydrate molecular recognition: a spectroscopic investigation of carbohydrate-aromatic interactions

The physical basis of carbohydrate molecular recognition at aromatic protein binding sites is explored by creating molecular complexes between a series of selected monosaccharides and toluene (as a truncated model for phenylalanine). They are formed at low temperatures under molecular beam conditions, and detected and characterized through mass-selected, infrared ion depletion spectroscopy-a strategy which exploits the extraordinary sensitivity of their vibrational signatures to the local hydrogen-bonded environment of their OH groups. The trial set of carbohydrates, alpha- and beta-anomers of glucose, galactose and fucose, reflects ligand fragments in naturally occurring protein-carbohydrate complexes and also allows an investigation of the effect of systematic structural changes, including the shape and extent of 'apolar' patches on the pyranose ring, removal of the OH on the exocyclic hydroxymethyl group, and removal of the aglycon. Bound complexes invariably form, establishing the general existence of intrinsic intermolecular potential minima. In most of the cases explored, comparison between recorded and computed vibrational spectra of the bound and free carbohydrates in the absence of solvent water molecules reveal that dispersion forces involving CH-pi interactions, which promote little if any distortion of the bound carbohydrate, predominate although complexes bound through specific OH-pi hydrogen-bonded interactions have also been identified. Since the complexes form at low temperatures in the absence of water, entropic contributions associated with the reorganization of surrounding water molecules, the essence of the proposed 'hydrophobic interaction', cannot contribute and other modes of binding drive the recognition of sugars by aromatic residues. Excitingly, some of the proposed structures mirror those found in naturally occurring protein-carbohydrate binding sites.